1. The Field of the Invention
This invention relates to signal processing of light waves and other electromagnetic radiation and, more particularly, to novel systems and methods for detection and use of coherent photonic signals in various applications.
2. Background
Coherence detection using interference is an important element of signal processing for optical signals. In general, when a signal is to be detected, the detection process relies on transmission and receipt of a signal having a value a substantial distance from a value of some base noise level. In order to detect a signal, some window of bandwidth at which the signal is expected to occur will be selected. In order to provide more channels of data, it is desirable to be able to narrow down the bandwidth that is required to receive a particular signal.
Broadcasting or transmitting a signal precisely, with a minimum of noise at other frequencies, is important. Likewise, filtering and detecting a received signal over a narrow band, despite any associated noise, is important for communication. Narrowing the bandwidth of operation of a receiving apparatus requires a filter. Such a filter requires, in the case of optical systems, detection of the coherence of a signal using interference, and thus the applicability of that signal to the frequency range of interest.
As the relative phase of two coherent signals changes, the difference between the constructive interference (CI) and destructive interference (DI) outputs of an interferometer reduces as the phase difference approaches 90 degrees. Thus, a dead spot exists when differential detection is used, and when the two signals are out of phase by 90 degrees. Thus, coherence detection is phase-sensitive. What is needed is a method and apparatus for phase-insensitive coherence detection.
In view of the foregoing, it is a primary object of the present invention to provide a method and apparatus for phase-insensitive coherence detection. It is another object of the invention to account for the dead spot that occurs when the phase difference is 90 degrees. It is another object of the invention to avoid any dead spot in the bandwidth of a coherence detector by modifying the input to an interferometer. It is another object to avoid a dead zone or dead spot when the phase difference is 90 degrees by modifying the output of an interferometer.
Further objects of the invention include providing a phase and frequency insensitive detection of coherence in photonic signals. It is yet a further object of the invention to provide a sensor for telecommunications lines, for receiving photonic signals, narrowing the required bandwidth necessary for effective capture of a received signal. It is another object of the invention to provide various apparatus implementing coherence detectors therein, for example: spectrum analyzers, signal processors, and so forth. Another object is to expand bandwidth for greater selectivity.
Consistent with the foregoing objects, and in accordance with the invention as embodied and broadly described herein, a method and apparatus are disclosed in one embodiment of the present invention provide multi-domain, phase-compensated, differential-coherence detection of photonic signals for interferometric processes. Manufacture of devices holographically and repeatably is done with emulsion development in situ or with removablek, automatic registration structures connecting and registering holograms and photonic sources with respect to each other in a single frame.
Photonic or electronic post processing may include outputs from a cycling or rotation between differently phased complementary outputs of constructive and destructive interference. A hyper-selective, direct-conversion, expanded-bandpass filter may rely on an expanded bandpass for ease of filtering, with no dead zones for zero beat frequency cases. A hyper-heterodyning, expanded bandpass system may also provide improved filtering and signal-to-noise ratios. An ultra-high-resolution, broadband spectrum analyzer may operate in multiple domains, including complex xe2x80x9cfingerprintsxe2x80x9d of phase, frequency, and other parameters.
The associated technologies of the invention may be used to produce extreme precision in multi-domain locking of sophisticated waveforms varying in several domains. Phase-masking techniques may provide phased arrays of complementary outputs over a broad band, such as may be implemented in a projected phase-mask, multiple phase interferometer. Topographic holographic imaging and projection techniques are enabled at very fine resolutions, while minimizing required information for systems such as holographic television. Phase-stabilization, modulation, compensation and the like are enabled by devices and methods in accordance with the invention, and may be servo-controlled.
Coherence detection may rely on an interferometer called a homodyne. A homodyne may require a single interferometer having sensors such as photodiodes, or other elements for detecting the light signal output, and forwarding a communications signal to a device. In a homodyne, adjustment typically provides for one sensor xe2x80x9cdetectorxe2x80x9d to receive energy from a region of destructive interference xe2x80x9cDIxe2x80x9d of two photonic beams. Another region may provide an area of constructive interference xe2x80x9cCIxe2x80x9d due to an interference pattern between the two photonic beams.
When two photonic inputs into an interferometer are coherent, two outputs provide a differential with respect to one another. If non-coherent light arrives as inputs, then outputs to the two sensors or detectors will lack the pronounced differential, and may effectively be non-differentiable. A differential detector for measuring the overall difference between the two signals received at the two sensors may thus determine if coherence exists. The existence of coherence can be used to indicate that a signal at a desired or expected frequency is arriving at the detectors to be processed.
Within contemplation is an embodiment of an apparatus in accordance with the invention in which a portion of a differential output provides a feedback signal to a servo circuit. This servo circuit controls an electrically driven or control phase-adjusting optical element in a photonic input pad leading to an interferometer. In one embodiment, the servo mechanism so constructed can change the phase of an input signal to avoid any dead spot near the 90 degree or quarter wave zone. As a result, any phase change that occurs between two inputs may be tracked by a servo in order that the differential output of an interferometer will be continuously adjusted to avoid any dead zone or dead spot condition.
In an alternative embodiment, the 90 degree or quarter-wave dead spot may be avoided in an output signal by providing at least two interferometers energized by a shared input signal. Accordingly, one input of one of the interferometers may be optically phase shifted so that at least one of the interferometers provides a differential output when the two inputs are coherent. The two differential outputs may then be combined into a single, phase-insensitive output.
In one embodiment, coherence detection may be implemented in a narrowband active optical filter or photonic active filter. A signal selection process may be useful in a demultiplexer, such as a wave division multiplexer (WDM) or a time division multiplexer (TDM). Coherence detection elements based on interference between a detected incoming signal, and a reference signal, may provide extremely narrowband selection allowing a significant increase in the channel-carrying capacity of an optical communication system.
In certain embodiments, a coherence detector implemented as a filter in a wavelength demultiplexing system may be used for precise wavelength measurements, thus forming a spectrum analyzer. A phase-insensitive method and apparatus for coherence detection is essential, and may be accomplished by splitting an input signal, and a reference signal, into a number of individual beams, each having substantially equal intensity, but different directions of propagation.
The beams may then be recombined using beam combiners, such as certain types of beam splitters, and directed along a shared optical path. The light intensity in each channel may be detected by a detector such as a photodiode or other appropriate sensor. Ultimately, output signals from each sensor may be compared in a differential circuit. When multiple interferometers are used, multiple pairs of sensors are provided.
Each pair of sensors provides a differential output. These differential outputs are then combined electrically to provide a coherence status outputs signal. The interferometers are organized as discussed above to provide at least one differential output whenever the phase difference between the input and reference signals is within 90xc2x0 relative phase values of 0, 90, 180, or 270 degrees. These differential status outputs result whenever coherence is achieved, regardless of the relative phase. By covering the full range of 360xc2x0, the usual dead spots are eliminated.
By appropriate selection of a frequency between a reference signal and an incoming signal, one may achieve a condition wherein all channels of a multiplexed or other system have different light intensities. Each intensity corresponds to a particular value of an initial phase of an incoming signal. In such a case, an output signal from a differential circuit may be obtained, provided that the oscillation rate of an interferometric pattern is within the bandwidth of a particular detector, such as a photodiode.
Additional details in certain embodiments, provide a method of phase-insensitive coherence detection may include providing two beams of electromagnetic energy, producing interference between a portion of the first beam and a portion of the second beam in an interferometer. Outputs of the interferometer may have a relative differential when the beams are coherent, and have a phase difference other than a quarter-wave position, or a 90 degree phase difference. Meanwhile, a method in accordance with the invention may produce interference between a second portion of the original beam, and a phase shifted portion of the second beam, through a second interferometer. Outputs of the second interferometer may have a relative differential when the beams are coherent, and have a phase difference other than a quarter-wave position, or a 90 degree phase difference.
In one presently preferred method in accordance with the invention, energy may be detected from the first and second photonic signals, using a first differential detection means to provide a first differential signal, and using a second differential detection means to provide a second differential signal. Thereafter, the first and second differential signals may be combined to provide a coherence detection output or a status detection for the coherence of the first and second photonic signals. Accordingly, the output may change when the first and second beams are coherent, regardless of any phase difference between the first and second photonic beams originally input.
In one embodiment, a method and apparatus in accordance with the invention may stabilize coherence detection by providing first and second beams of electromagnetic energy, and directing the second beam through a servo-controlled phase adjustment mechanism in order to provide a phase-correct beam. Thereafter, interference may be produced between the first beam and the phase corrected beam in an interferometer in order to produce a differential output when the first and second beams are coherent.
Detecting the differential signals may then provide at least one output in the feedback signal into a servo-controlled phase adjustment mechanism in order to adjust the phase of the phase-corrected beam. Accordingly, the condition is avoided wherein the phase difference between a phase-corrected beam and the original first beam is ever 90 degrees. Accordingly, the differential levels are stabilized, eliminating any singularity (dead zone) at the 90 degree or quarter-wave difference position. Thus, coherence detection is provided in a phase-insensitive way.
In certain embodiments, an apparatus and method in accordance with the invention may provide extremely high resolution phase comparison of numerous photonic signals, simultaneously. Such a mechanism and method are possible in conjunction with a two-dimensional phase mask, a two-dimensional beam splitter, a two-dimensional lens matrix, a two-dimensional sensor matrix, or the like. In certain embodiments, parallel processing of photonic spectra may be provided through numerous paralleled channels. Numerous sets of double channels may be provided or large sets of small groups of channels may be provided. A very fine, almost infinitesimally fine, resolution of a single channel or a single set of channels may be approachable.
Application of the methods and apparatus in accordance with the invention to broadband applications may depend on the bandwidth of available photonic or other wave-type reference sources. For example, reference sources may be in the visible spectrum, infrared, ultra violet (UV), acoustic, or the like. The ratio of the size of an aperture to a particular wave length being used may effect the bandwidth of applicability to an apparatus and method in accordance with the invention.
In one embodiment, an apparatus and method in accordance with the invention may operate over multiple sets of dual channels. The sets of dual channels may each have a coherence status that may be detected individually. In one embodiment, an extremely fine resolution of coherence status may be determined for many pairs of channels. Alternatively, sets of channels may have more than two channels, but may still have extremely fine resolution of coherence. In certain embodiments, certain sets of channels can be of the same frequency, or sets of channels may be at different frequencies.
When the reference and input signals have different frequencies, the combined waves in the interferometer, or interferometers, as appropriate, naturally sequence through 360xc2x0 of phase differences at a beat frequency rate. A method and apparatus in accordance with the invention are so organized as to exploit this phenomenon. By arrangement of the phase-adjusted interferometers, the energy of constructive interference, and its destructing interference complement, along with the energy differential presented to any sensor, sequences the energy through the set of sensors. The result is a multiplying of the beat frequency for all signals that are not zero-beat with respect to the reference.
In addition, an absolute value and other frequency multipliers can be used. This provides an expanded bandpass that may render easier subsequent filtering. The entire receiver can be made more selective. The term hyper-resolution has been applied to the resulting increase (improvement) in the degree of resolution, and consequent increase in selectivity.
Applications for detection in accordance with the invention may include molecular spectroscopy, pharmaceutical identification of compositions, and resolution of astronomical emission spectra. Increased subdivision of signal bandwidth may greatly augment wave-division multiplexing.
Coherence detection in accordance with the invention may be used for high speed identification of the emission spectra of exhaust plumes from rockets or missiles. A scanner or detector for interference may rely on coherence detection in accordance with the invention. Other applications may include echo location in wave-transmitting media, whether ultra sonic, audible, or other sonar ranges. Medical sonographic data collection and analysis, including ultrasound detection, ultrasound imaging, dynamic signal processing imaging, dynamic signal processing, post processing analysis, spectra analysis, spacial analysis, or the like may be provided. Reflectometry, or Time-Delay Reflectometry (TDR), precise analysis in real time of TDR data, may rely on coherence detection in accordance with the invention.
Frequency locking of one or more wave sources with respect to a stationary reference wave source, whether an oscillator or frequency standard, may provide numerous advantages and much higher speeds using photonic coherence detection. Frequency locking of one or more wave sources to a non-stationary wave source, such as may be applied to frequency tracking, FM demodulation, frequency monitoring, frequency stabilization, Doppler shift tracking, and the like may also benefit from a filter system corresponding to an apparatus is accordance with the invention.
Phase locking of one or more wave sources to a stationary wave source in a light spectrum, such as a laser mode locking apparatus is also contemplated. Likewise, another application is phase locking of one or more wave sources to a non-stationary electromagnetic wave source, such as a phased-locked loop, FM demodulation, phase monitoring, or phase tracking may rely on coherence detection systems in accordance with the invention.
Likewise, parallel processing of information generated by non-photonic sources, such as seismic data processing, as well as sonar, radar, and other information processing may rely on coherence detection systems in accordance with the invention. Dynamic noise emission analysis with respect to spatial locations, spectral analysis, and active or dynamic noise cancellation processes may be executed at sufficiently high speeds using photonic coherence detection in accordance with the invention. For example, active or automatic noise-emission reduction for automobiles, aircraft, and the like are contemplated. Similarly, engine noise may be abated by tracking an active reduction by servo mechanisms, providing precisely-selected frequencies, according to the change in frequencies of such noise-producing elements as engines, turbines, and the like.
In summary, various embodiments of apparatus and methods in accordance with the invention may provide for detection of coherence in multiple domains for a waveform, and using the lack of or presence of coherence to perform a multiplicity of useful functions. Some of those functions include phase-insensitive coherence detection, multi-domain differential coherence detection, holographic manufacture in-place for lenses and holograms in order to maintain more precise registration of components, and various types of electronic and photonic signal processing and post-detection processing. Also available are functions including hypersensitive bandpass filtering at zero beat frequency, such as the hyper-selective, direct-conversion filtering apparatus and method. Hyper-heterodyning, expanded bandpass apparatus and methods are also available. Hyper-resolution, broadband spectrum analyzers and multi-dimensional, photonic waveform fingerprint analyzers are also contemplated. The technology may also produce a frequency-locked photonic loop, a phase-compensated coherence detection interferometer and a multiple-phase-mask interferometer with a broadband phase mask, relying on a projected phase mask.